peer reviewed ; Hydroxyurea (HU) is considered to be the most successful drug therapy for severe sickle cell disease (SCD). Nevertheless, questions remain regarding its benefits in very young children and its role in the prevention of cerebrovascular events.There were 127 SCD patients treated with no attempt to reach maximal tolerated doses who entered the Belgian Registry:109 for standard criteria and 18 who were at risk of stroke only. During 426 patient-years of follow-up for patients with standard criteria, 3.3 acute chest syndromes, 1.3 cerebrovascular events, and 1.1 osteonecrosis per 100 patient-years were observed.A subgroup of 32 patients followed for 6 years experienced significant benefit over this period. In each subgroup of children (younger than 2 years, 2-5, 6-9, and 10-19 years) followed for 2 years,clinical and biologic changes were similar,except for children younger than 2 years who had no total hemoglobin increase and remained at risk of severe anemia. In 72 patients evaluated by transcranial Doppler studies (TCD), 34 patients were at risk of primary stroke and only 1had a cerebrovascular event after a follow-up of 96 patient-years. These results confirm the benefit of HU, even in very young children, and its possible role in primary stroke prevention.